The present invention generally relates to a Viterbi decoding method and a Viterbi decoding apparatus. More specifically, the present invention is directed to such Viterbi decoding method/apparatus operable under 6-value/4-state condition, capable of performing most likelihood decoding.
In digital data transmissions, signal processing operation by combining the partial response (PR) equalization with the Viterbi decoding is carried out. In the partial response equalization, codes interference is applied to waveforms to thereby perform effective transmissions. In the Viterbi decoding, the most certain data series is selected from the previously correlative data series to be reproduced.
For instance, in a magneto-optical disk and a phase-change disk, a modulation of RL min (minimum inverse width)=2 such as (1, 7) RLL (Run Length Limited)+NRZI (Non Return to Zero Inverted) is utilized. The 3-value/4-state conditioned Viterbi decoding in which PR (1, 1) is used as the equalization characteristic, or the 4-value/4-state conditioned Viterbi decoding in which PR (0.5, 1, 0.5) is used as the equalization characteristic is carried out. In the respective Viterbi decoding methods, the 0.5 T0 bits-shifted point is applied as the discrimination point.
However, in the actual recording/reproducing system, the above-described Viterbi decoding can be hardly made coincident with the desirable equalization characteristic. Also, since the S/N of the discrimination point becomes maximum having the intermediate characteristic of PR (1, 1) and PR (0.5, 1, 0.5) in response to the characteristic of the recording/reproducing system, it is not strictly equalized to any one of the equalization characteristics. As a consequence, the reproduction signal is deviated from the reference amplitude level of the Viterbi decoding due to the shifts from these equalization characteristics. Thus, there is a problem that the error rate is deteriorated.
Also, in the case that the Viterbi decoding is applied, the equalized reproduction signal must be A/D-converted in response to the PLL clock. However, for instance, in such a case that the recording rates are different from each other, depending upon the zones, as in the multi-zone CAV, it is practically difficult to precisely manage the phases of the PLL clocks. Therefore, this may cause that the reproduction signal is shifted from the reference amplitude level of the Viterbi decoding. Accordingly, there is another problem that the error rate is deteriorated.
Furthermore, in thermal recording operations, such a phenomenon will occur that a reproduction signal is made asymmetry with respect to an upper signal portion and a lower signal portion, because of excessive/short power of recording laser. This asymmetry cannot be compensated by a linear equalizer. Also, in any one of the Viterbi decoding operable under 3-value/4-state and 4-value/4-state, symmetry of a reproduction signal constitutes an initial condition. This phenomenon may also cause that the reproduction signal is shifted from the reference amplitude level of the Viterbi decoding. There is another problem that the error rate is deteriorated.